Anaerobic digester system for animal waste stabilization and biogas recovery

ABSTRACT

An ambient anaerobic digester system for anaerobic digestion of animal waste with biogas production and recovery is provided. The anaerobic digester system includes a substantially flexible bladder for anaerobically digesting the animal waste with biogas production and transmitting the biogas to at least one biogas storage container, biogas use device or a combination thereof. In a preferred form, the substantially flexible bladder has a waste inlet, a digester effluent outlet, one or more sludge access ports and a biogas outlet in a top surface thereof. 
     The anaerobic digester system may inexpensively, simply, reliably, and effectively be used to treat animal waste without energy expenditure and little capital expense, thus minimizing pollution typically caused by animal waste.

FIELD OF THE INVENTION

This invention relates to pollution control and more specifically, to animproved anaerobic digester system for animal waste treatment withbiogas production and recovery.

BACKGROUND OF THE INVENTION

Livestock confinement facilities generate large amounts of animal wastethat can create serious environmental and human health concerns. Forexample, animal waste constituents such as organic matter, nitrogen,phosphorus, pathogens and metals can degrade water quality, air quality,and adversely impact human health. Organic matter, for example, containsa high amount of biodegradable organics and when discharged to surfacewaters will compete for, and deplete the limited amount of dissolvedoxygen available, causing fish kills and other undesirable impacts.Similarly nutrient loading from nitrogen and phosphorus can lead toeutrophication of surface waters. Thus, in the United States through theClean Water Act and in other developed countries, animal waste may notbe discharged but terminally applied to land as a supplement tocommercial fertilizer. These requirements do not exist in many othercountries having large numbers of livestock and thus, animal wasteadversely impacts environmental quality. For example, there is growingevidence of water pollution caused by the discharge of livestock wasteinto surface waters in various global watersheds and there is nowevidence of these discharges affecting coastal water quality such as inthe South China Sea, the Gulf of Thailand, and in the Gulf of Mexico.

Animal wastes also impact air quality, which include odor and greenhousegas emissions. Wastes also contain viruses, bacteria, protozoa, andhelminthes that when transmitted to humans can adversely impact humanhealth in a number of ways some of which are life threatening.

A variety of technical approaches have been used to abate these concernsto varying degrees. At the most basic level, wastes are stored and landapplied at agronomic rates to reduce nutrient loading and run-offpotential. However manure storage does not stabilize waste and thereforedoes not reduce odor, pathogens, or oxygen demanding materials. Morecomplex processes may use a combination of pre-treatment, primary,secondary, and tertiary treatment processes to provide comparativelysuperior levels of managing environmental and human health relatedconcerns. Primary treatment is an essential first step when secondaryand tertiary processes are considered as primary treatment reducesoxygen demand, reduces pathogens, converts nitrogen and phosphorus intoplant available forms, specifically ammonia N and phosphate. Plantavailable forms of nutrients ensure uptake with a high level ofpredictability when applied at agronomic rates relative to crop type.

Typically anaerobic processes are used in primary treatment of livestockand other high strength organics as they are economically desirable whencompared to aerobic methods. Anaerobic processes transform manure into avariety of end products, including digester effluent and biogas. Variousanaerobic systems have been used commercially depending on livestocktype, climate and water usage.

One of the most common anaerobic systems used for the treatment ofdilute manure is an anaerobic treatment lagoon. In lagoons or any otherunmixed systems, materials stratify into solid and liquid components.Sludge (biologically degraded solids) accumulate at the bottom of thelagoon and is composed of settled non-biodegradable and fixedconstituents of manure, and active and dead microbial cells. Sludge isblack, moderately viscous, typically about 10 percent solids and 90percent liquid, and high in nutrients, bacteria, and organic matter.Sludge is the byproduct of biological anaerobic degradation or thebiodegradable component of organic material. Sludge can be removedmanually or by pumps designed for higher solids applications i.e., 10 to15 percent solids.

The layer above the sludge is the liquid layer. This liquid, thedigester effluent, is low in solids (generally 0.3 to 0.6 percentsolids), moderately rich in nutrients and easily pumped with irrigationpumps. If the liquid and sludge are mixed, the solids content will rangebetween <1 percent and 8 percent solids, depending on the proportion ofprocess water, rainfall and sludge in the system. The digester effluentand sludge will contain all of the remaining (that which is notvolatilized to air) nitrogen, phosphorus, potassium, micronutrients, andmetals in the original manure. These can be further processed or landapplied.

Unfortunately, anaerobic treatment lagoons while effective instabilizing organics, are open systems and can emit odor, volatileorganic compounds (VOC's), and a number of other constituents into theair that are of growing concern. These gases consist of methane, agreenhouse gas with a warming potential 23 times that of carbon dioxide;ammonia and VOC's which are prerequisite gases in the formation of fineparticulate matter (smog), and hydrogen sulfide, an odor compound, whichcan also cause death in high concentrations. However, when this biogasconsisting of about 70% methane is captured in various types ofanaerobic digesters and utilized for its energy value, it can providefinancial benefit by offsetting energy costs while reducing the airimpacts by various combustion processes that destroy methane andhydrogen sulfide.

This biogas can be burned for heat or used to fuel an electric generatoramong other uses. The heat and electricity can be used on the farm orsold to others. As used herein, a “continuous biogas system” refers tothe continuous feeding of biogas to a biogas combustion device such asflares and engines for operation thereof. A “stored biogas system”refers to the storage of biogas for intermittent combustion and use.

There are a myriad of anaerobic digester systems and scales in usearound the world. These include simple unheated systems such as coveredlagoons and more complex systems that are heated to about 100° F. orhigher. Maintaining higher constant temperature reduces reactor volumesrequired to treat and stabilize waste. A conventional anaerobic digestersystem generally includes the following components: manure transfer andmixing pit, a digester made of steel, fiberglass, concrete, earth orother suitable material (including heating and mixing equipment ifneeded), biogas handling and transmission, and gas end use (combustion)equipment such as electric generation equipment. Conventional anaerobicdigesters can also require significant operational oversight dependingon operational mode and temperature. Conventional anaerobic digestersystems also require proper design and sizing to maintain criticalbacterial populations responsible for waste treatment and stabilizationfor sustained long-term predictable performance. Sizing requirements arebased on hydraulic retention time (HRT), and loading rate where theoperating temperature affects these sizing parameters. These factors(size, materials, operational requirements) affect digester costs, whichmay be fairly capital intensive and in some economies and farm scalesmay not be affordable or may be inoperable if experienced techniciansare not available.

The issues of affordability and operational ability are exacerbated indeveloping countries or countries with economies in transition. Thesecountries are predominantly located between 35° north and south latitudewhere the range of farm scales may be very small household farms to verylarge corporate production oriented types of farms. This range mayencompass farm scales of 5–100,000 pigs or 1–10,000 milk cows perfacility. These regions are also showing signs of severe environmentaldegradation, particularly water and human health, due to largepopulation growth and concentration. These regions to various extentshave promoted various anaerobic digesters operated at ambienttemperatures constructed from an array of locally available materials tocontrol costs and more expensive and operationally complex systems forlarger scale farms. In many cases these smaller systems use low qualitymaterials of limited durability and lifetime to control cost and thelarger scale systems may not be cost effective or transportable.Moreover the sizing methodologies used predominantly for smaller scalesystems are based on reactor volumes to meet the daily gas requirementsfor a household or farm (biogas is about 20 cu. Ft/person/day). Thisapproach typically results in only partially stabilizing the wastestream as bacterial populations are subject to washout, shortcircuiting, and/or excessive loading rates. Furthermore these systemsaccumulate solids with no provision for removal. Solids and/or sludgeaccumulation reduce reactor volume and HRT and increase the loading ratecausing decreased gas production, increased CO₂ concentration in gasstream and/or system failure. The operating track record of thesesystems has shown marginal to poor performance. While the need toprovide affordable technology in the marketplace is essential, it isalso essential to integrate quality with performance that enhances theenvironmental and sanitary conditions for both human and animal health.

Accordingly, there has been a need for a novel improved anaerobicdigester system and method for treating animal waste that arepredictable, effective, durable, affordable, simple to operate,portable, labor efficient, environmentally friendly, and substantiallyreliable year-round in tropical and semi-tropical regions locatedbetween 35° north and south latitude where these areas have averageambient temperatures of about 65° F. or higher (when at sea level orslightly higher) for passive heating of the digester. There is a furtherneed for a novel improved anaerobic digester and method for primarywaste treatment and biogas production for the small, medium, and largescale farms. There is a still further need for a novel anaerobicdigester system and method that may be combined with secondary andtertiary processes which increases its environmental performancerelative to air, water and human health quality. There is an additionalneed for a novel improved anaerobic digester system and method that helpcontrol air and water pollution from livestock waste, protect publichealth and offer an opportunity for the waste to be used as a renewableenergy resource. The present fulfills these needs and provides otherrelated advantages.

SUMMARY OF THE INVENTION

The present invention is concerned with an anaerobic digester systemcomprising a substantially flexible bladder for anaerobically digestinganimal waste, with biogas production and recovery. The substantiallyflexible bladder has one or more waste inlets, digester effluentoutlets, sludge access ports, and biogas outlets on a top surfacethereof. The anaerobic digester system may further comprise one or morebiogas storage containers for receiving biogas from the substantiallyflexible bladder. The bladder and the one or more biogas storagecontainers may be constructed with reinforced geo-membrane material. Thebladder may be sized to maintain critical bacterial populations forareas located between 35° north and south latitude where ambienttemperatures are about 65 degrees Fahrenheit or higher and toaccommodate varying waste volumes from livestock farms. Thesubstantially flexible bladder may be portable, factory fabricated andfield installed.

The bladder may include an internal baffle defining a U-shaped interiorhaving an inlet side and an outlet side. The waste inlet and the one ormore sludge access ports are defined in the inlet side of the bladderand the digester effluent port and one or more biogas outlets aredefined in the outlet side of the bladder. Alternatively, the wasteinlet and digester effluent outlets may be at opposite ends of thebladder.

The animal waste enters the bladder at the waste inlet and flows throughthe bladder and undergoes bacterial digestion before exiting through thedigester effluent outlet for further processing or land application. Thebladder, for primary waste treatment, may be complemented by otherstructures for secondary and tertiary waste treatments.

The one or more sludge access ports may be used to withdraw sludge fromthe bladder. A sludge drawdown tube may extend inwardly into thesubstantially flexible bladder from each of the sludge access ports. Thesludge may be separate from the liquid fraction (i.e. the digestereffluent) and either manually or mechanically removed through the one ormore sludge access ports.

The biogas generated in the bladder from anaerobic digestion of theanimal waste may be discharged through the biogas outlet to a biogaspipe for transmission to one or more biogas use devices and/or one ormore biogas storage containers. Pressure within the bladder may beregulated through a manometer. A valve regulates the flow of biogas outof the bladder and into and out of the one or more biogas storagecontainers.

Where biogas may be sold, distributed and used off-site, the biogasstorage containers may be portable. Alternatively, the biogas may bestored inside the bladder for use onsite. When used in this manner, thesystem further comprises an external displacement tank. The externaldisplacement tank is designed to hold waste that is displaced byinternal bladder pressure. As biogas is produced inside the bladder, thewaste is pushed out of the digester effluent outlet into the externaldisplacement tank and when biogas is used, the displaced waste flowsback into the bladder through the digester effluent outlet.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a perspective view of a substantially flexible bladderembodying the invention, illustrating the bladder having a waste inlet,a sludge access port, a digester effluent outlet, and a biogas outlet ina top surface thereof;

FIG. 2 is a perspective view of an exemplary manometer for the bladderof FIG. 1;

FIG. 3 is an interior perspective view of the bladder of FIG. 1,illustrating the bladder in dotted lines to show the interior thereofwith a baffle between an inlet and an outlet side of the bladder and asludge drawdown tube extending inwardly from the sludge access port anda substantially leak-proof cap thereon;

FIG. 3A is another interior operational perspective view of the bladderof FIGS. 1 and 3, illustrating by arrows the movement of animal waste(not shown) into the waste inlet and out the digester effluent outletand release of biogas through a biogas outlet pipe connected to theexemplary manometer of FIG. 2;

FIG. 3B is a similar view to FIG. 3A, illustrating transmittal of thebiogas to an exemplary biogas storage container;

FIG. 3C is a top schematic view of FIG. 3A;

FIG. 4 is a perspective view of an alternative configuration of thesubstantially flexible bladder;

FIG. 4A is an interior operational perspective view of the bladder ofFIG. 4, illustrating by arrows the movement of animal waste into thewaste inlet and out the digester effluent outlet and release of biogasthrough the biogas outlet pipe connected to the exemplary manometer ofFIG. 2 and removal of sludge through the sludge drawdown tube and sludgeaccess port;

FIG. 5 is a schematic of an exemplary anaerobic digester system,illustrating use of the substantially flexible bladder as the primarywaste treatment;

FIG. 6 is a perspective view of an exemplary biogas storage containercoupled to a ball valve, illustrating by arrows the transmission ofbiogas into and out of the biogas storage container;

FIG. 6A is another perspective view of the exemplary biogas storagecontainer of FIG. 6 with a reinforcing wafer in a top portion thereof;

FIG. 7 is a top view of a supporting platform with flexible supports forstoring a plurality of exemplary cylindrical biogas storage containers;

FIG. 8 is a perspective view of a plurality of exemplary stackedsubstantially rectangular biogas storage containers, illustrating theremoval of biogas therefrom;

FIG. 9 is a perspective view of an alternative embodiment of theanaerobic digester system, illustrating a floor in dotted lines with thesubstantially flexible bladder under the floor and a displacement tankand inlet basin in fluid communication therewith;

FIG. 9A is a side view of the anaerobic digester system of FIG. 9; and

FIG. 10 is an assembly view of a 3-way ball valve connecting to thebiogas outlet in a substantially flexible bladder or biogas storagecontainer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings for purposes of illustration, the presentinvention is concerned with an improved anaerobic digester system foranaerobic digestion of animal waste with biogas production and recovery,the system generally designated in the accompanying drawings by thereference number 10. The anaerobic digester system comprises, generally,a substantially flexible bladder 12 and 120 constructed of a reinforcedgeo-membrane material for anaerobically digesting waste with biogasproduction having one or more waste inlets 14, digester effluent outlets16, sludge access ports 18, and biogas outlets 20 in a top surface 22thereof and sized to have a design operating volume based on latitude tomaintain the greater of a selected maximum daily volatile solids (VS)loading rate per 1,000 ft³, or the minimum hydraulic retention time(HRT) adequate for methane production. The anaerobic digester system 10may further comprise one or more biogas storage containers 24 and 240for receiving biogas from the substantially flexible bladder 12 and 120.

The bladder 12 or 120, for primary waste treatment, biologicallystabilizes the animal waste. Biogas production is a byproduct of theanaerobic process. The biogas may be recovered as a renewable energyresource or delivered to combustion devices as an energy source or forair quality objectives such as flaring for odor or greenhouse biogasemission control to protect air quality.

As shown in FIG. 1, the bladder 12 and 120 comprises a substantiallyflexible bag having the top surface 22 and a bottom surface 26. Thebladder 12 and 120 may be constructed of reinforced geo-membranematerials having the following approximate physical properties:

PHYSICAL PROPERTY TEST METHOD STANDARD Thickness ASTM D751 20–30 milsminimum Tear Strength ASTM D4533 35 lbf minimum Trapezoid Tear BreakingYield Strength ASTM D751 550/550 Lbf Grab Tensile Dimensional StabilityASTM D1204 1% maximum each 212° F.-1 hr direction Adhesion Heat SealedASTM D751 35 lb_(f)/2 in minimum Seam Dielectric Weld Dead Load SeamShear MIL-T-52983E 2 in seam, 4 hrs, Strength (modified), Para. 1 instrip 4.5.2.19 210 lb_(f)@70° F. 105 lb_(f)@160° F. Bursting StrengthASTM D751 650 lb_(f) Typical Ball Tip 800 lb_(f) Typical Adhesion-PlyASTM D2413 Puncture Resistance ASTM D4833 50 lb_(f) Typical TearingStrength, lbf ASTM D5884 55 min Tongue Tear Ozone Resistance, 100 ASTMD1149 No cracks pphm, 168 hours Resistance to xenon-arc ASTM G155 Nocracks weathering¹ 0.70 W/m² No loss of breaking or 80° C. B.P.T.tearing strength ASTM = American Society for Testing and Materialsincorporated herein by reference ¹Approximately equivalent to 8000 hoursexposure at 0.35 W/m² irradiance

Suitable materials for the bladder include reinforced geo-membranematerials such as XR-5® 8130 or XR-3® 8228 reinforced geo-membranesavailable from the Seaman Corporation, Wooster, Ohio, and reinforcedgeo-membranes from Cooley Engineered Membranes, Pawucket, R.I. or thelike. The materials may be reinforced with scrim material or the like.The material should be durable (puncture resistant), with highdimensional stability (about 1% maximum in each direction), and UVresistance. The minimum thickness for the geo-membrane material is about20 mil.

The bladder may be factory fabricated with dielectric or wedge weldingmethods or similar type method to produce quality, non-leaking seams 28.The seam strengths may be within about 5–10% the tear strength of thematerial for a substantially gas tight seal. The bladder may befabricated from materials that are compatible with such dielectric orwedge welding technologies.

The shape and size of the bladder may be limited by manufacturingconcerns. A substantially rectangular, pillow-shaped bladder 12 is shownin FIGS. 1 and 3–3B. The bladder may also be substantially cylindricalas shown in FIGS. 4–4A. While not wishing to be bound to any shape orsize, the bladder may be manufactured to maximize the distance betweenthe waste inlet and the digester effluent outlet. The bladder 12 and 120may have about a 3 to 1 to about a 5 to 1 length/width ratio, preferablya 3:1 length: width ratio.

The bladder 12 and 120 is sized to maintain critical bacterialpopulations and to accommodate varying waste volumes from livestockfarms. The sizing is based on parameters such as Hydraulic RetentionTime (HRT) and Loading Rate. These are determined by seasonaltemperature. Waste must be held in the digester for a period of time fordigestion to occur. Hydraulic Retention Times may be increased dependingon uncertainties of waste volume encountered at the livestock farmsand/or seasonal fluctuations in local ambient temperature. If the localambient temperature is lower than about 65 degrees Fahrenheit, hydraulicretention times may have to be increased. The ambient anaerobicdigestion bladder may be sized based on the following table to maintainthe bacterial population to treat and stabilize wastes where the designoperating volume shall be based either on the maximum daily volatilesolids (VS) loading rate per 1,000 ft³, or the minimum hydraulicretention time (HRT) adequate for methane production, whichever isgreater. The maximum daily VS loading rate and the minimum HRT may beselected from the values in Table 1 below according to latitude. Incases where systems may be located in between the specified latitudes,the higher latitude parameter may be selected.

TABLE 1 Latitude Min. HRT Max. Loading Rate (N° & S°) (days) lbs.VS/1,000 ft.³/day 35 40 10 30 35 12 20 26 18 10 24 20 0 23 22

There are two events that affect sizing: Volatile solids and processwater. Both of these can be variable with process water having the mostvariability. If water use is low, then the system is sized on loadingrate and is loading rate limited. If water use is high, then the systemwill be HRT limited and will be comparatively much larger than a loadingrate limited system. The more water used, the larger the volumerequirement. The formula for sizing the bladder is as follows:

A=Total VS Lbs. The value for A may be determined from a table such asthe exemplary Table 2 shown below for pigs in the United States or fromother representative methods including sampling and analysis ofmaterials.

TABLE 2 Animal Type LbsVS/day/1,000 lbs live weight Grower (40–220 lb)5.4 Gestating Sow 2.13 Lactating Sow 5.4 Boar 1.7 Nursery (0–40 lb) 8.8(USDA/NRCS Field Waste Management Handbook). It is to be appreciatedthat persons skilled in the art may make known reference to similartables to determine the value for

B=Total volume waste cubic feet/day

C=Cubic feet process water/day

Where:

(B+C)×HRT (for latitude)=D (total volume)

Then check loading rate by: A/D

If the loading rate exceeds maximum as set forth in Table 1, increaseHRT until condition is satisfied.

In a preferred form as shown in FIGS. 1 and 4, the bladder has one wasteinlet, one digester effluent outlet, one biogas outlet, and one or moresludge access ports. As shown in FIG. 3, the substantially rectangularbladder 12 may include an internal baffle 30 defining a U-shapedinterior having an inlet side 32 and an outlet side 34. The U-shapedinterior typically has a better footprint (more compact) for the farmswith space constraints or desire inlets and outlets on the same sidedependent on the configuration of the farm waste handling system. Asshown in FIG. 1, the waste inlet 14 and sludge access port 18 aredefined in the inlet side 32 of the bladder 12 and the digester effluentand biogas outlets 16 and 20 are defined in the outlet side 34 of thebladder. The waste inlet 14 and digester effluent outlet 16 may befitted respectively with a corresponding cap 36 as shown in FIG. 3 forprotection during shipment and to hold water and pressure during testingof the bladder. The caps may be removed when installed for use.

In the substantially cylindrical bladder 120 as shown in FIGS. 4 and 4A,the waste inlet 14 may be on a first end 38 with the digester effluentoutlet 16 on a second end 40 of the top surface 22. The biogas outlet 20may be substantially proximate the digester effluent outlet 16 as shownin FIG. 4A and the sludge access port 18 in substantially the center ofthe top surface 22 of the cylindrical bladder 120.

The waste inlet 14, digester effluent outlet 16, sludge access port 18,and biogas outlet 20 in the top surface of the bladder 12 and 120 makethem accessible for maintenance, etc. with the bottom surface 26 of thebladder on the ground or floor.

The one or more sludge access ports 18 may be used to withdraw sludgefrom the bladder 12 and 120. As shown in FIGS. 3 and 4A, a sludgedrawdown tube 42 may extend inwardly into the substantially flexiblebladder 12 and 120 from each of the sludge access ports 18. The sludgemay be separated from the liquid fraction (i.e. the digester effluent)and removed through the one or more sludge access ports 18 from the oneor more drawdown tubes 46. In smaller bladders with volumetriccapacities of about 3,000 ft³, there would typically be one sludgeaccess port with a corresponding sludge drawdown tube about 2 to about 3feet from the waste inlet or about ¼–⅓ down the length of the bladderfrom the waste inlet. As the volumetric capacity of the bladderincreases, the number of sludge access ports and corresponding sludgedrawdown tubes may increase. As the waste sits in the bladder, sludgeaccumulates at the bottom of the bladder. The sludge access port mayinclude an adapter (not shown) for receiving a flex hose (not shown)from a pump (not shown) to mechanically remove the solids.Alternatively, solids may be removed manually through the sludge accessport with a screened scoop/dipper or other simple hand operated removaldevice. The diameter of the one or more sludge access ports may be about2 to about 6 inches. A substantially gas tight cap 44 as shown in FIG. 3on the one or more sludge access ports 18 may be removed to manuallylift or pump out the sludge. Typically, the solids in cow (ruminant)manure would be separated from the liquid fraction prior to conveyancethrough the waste inlet 14 as cow manure solids float and may bypass thesludge drawdown tubes. For pig manure, the drawdown tubes may typicallybe used to draw off the sludge or solids.

The biogas generated in the bladder 12 and 120 from anaerobic digestionof the animal waste may be discharged through the biogas outlet 20 intoa biogas outlet pipe 46 for transmission to one or more biogas usedevices (not shown) and/or the one or more biogas storage containers 24and 240 as hereinafter described. The bladder 12 and 120 may bepressurized (weighted) by biogas production so biogas will flow withoutbiogas handling equipment such as blowers and compressors. Pressurewithin the bladder may be regulated through a manometer 48. Theexemplary manometer shown in FIG. 2 may be used and includes a firstflexible tube 50 filled with water and an attached bottle 52. The bottleis configured to self-regulate pressure by displacing water from anentrance connection allowing biogas to vent from a second tube 54extending above the water level. As pressure within the bladder falls,the water returns back into the manometer at a pre-selected pressure,about 24 inches water column. Although a manometer suitable for smallfarms has been described, it is to be appreciated that other manometertypes may be used within the confines of the invention. The manometermay come off the bladder 12 and 120 or a biogas storage container ashereinafter described

A ball valve 56 such as shown in FIG. 10 may regulate the flow of gasout of the bladder and into the biogas use device and/or into and out ofthe biogas storage container(s) as hereinafter described. The ball valvetypically remains in the open position. The ball valve may also serve asa condensate drain. The ball valve 56 includes a T-handle 58 and has aninlet and outlet 60 and 62 that may each be internally threaded as shownin FIG. 10 for threadably engaging with a male adapter 64, typically abushing. The inlet 60 of the ball valve may be coupled to a bulkheadfitting 66 in the biogas outlet of the bladder and/or biogas storagecontainer. The adapter 64 at the outlet 62 of the ball valve may becoupled to a nipple 68 for attachment to a gas hose (not shown) fortransmitting biogas to one or more biogas use devices. The valvematerials may preferably be constructed of non-corrosive materials, suchas PVC or the like. Although a 3-way ball valve is shown and described,it is to be appreciated that other types of valves may be used withinthe confines of the invention.

Biogas use devices (not shown) include flares, boilers, absorptioncoolers, engine generators, cook stoves, gas lighting or the like. Otherbiogas use devices are selected on a farm by farm basis dependent uponthe goals of the farmer i.e. odor control, electrical energy, heat,co-generation, cooking, lighting. The biogas may be delivered to thebiogas use device when the user turns on the device.

The biogas may also be stored in one or more external biogas storagecontainers 24 and 240. Stored biogas may be used on an intermittentbasis for cooking, lighting, heating or the like. An exemplary externalsubstantially cylindrical biogas storage container 24 is shown in FIGS.6 and 6A with a top portion and a bottom portion 70 and 72. The topand/or bottom portions may be structurally reinforced with a plywoodwafer 74 or the like to substantially prevent collapse of the containerwhen biogas is discharged. The wafer helps the biogas storage containermaintain its shape and provides for more even compression of the biogasstorage container. The wafer may be about ¼ inch to about ¾ inches thickand seam sealed in the top and bottom portions of the container. Aweight 76 such as a sandbag or the like may be supported on thewafer-reinforced top and bottom portions of the container to keepsubstantially constant positive pressure on the container to transmitgas. The container may further include a plurality of handles 78 toassist in the transport of the biogas storage containers as hereinafterdescribed. The handles may be affixed through the wafer and/or containermaterial.

The exemplary cylindrical biogas storage container 24 may be about 6feet high with a diameter of about 4 feet but other sizes and shapes maybe used within the confines of the invention. The external biogasstorage container may be constructed of the same or different flexiblegeo-membrane material as the bladder. The biogas storage container mayinflate with biogas production and deflate when biogas is used. Thebiogas storage container 24 has an opening 80 proximate the lower end ofthe container. The opening 80 is in fluid communication with the biogasoutlet in the substantially flexible bladder. The opening 80 may befitted with the bulkhead fitting 66 as shown in FIG. 10 for coupling thebiogas storage container 24 to the low pressure 3-way ball valve 56.

In an alternative embodiment, the one or more external biogas storagecontainers 240 may be substantially rectangular as shown in FIG. 8. Thesubstantially rectangular biogas storage containers may be stacked andeach includes the opening 80 in a top surface thereof for transmittingbiogas through an interconnected gas pipe 82 to the ball valve 56. Theweight 76 may be placed on the top surface of the uppermost container.The manometer 48 may be used with the biogas storage containers (notshown) to regulate pressure therein.

Where biogas may be sold, distributed and used off-site, the biogasstorage containers may be portable. When used in this manner, thehandles may be used to more easily transport the biogas storagecontainers. The biogas storage containers may be filled, transported,and refilled. This embodiment is particularly suitable for largedigester systems where biogas is transported for local use such ascooking and lighting. Although portable biogas storage containers havebeen described for use with the anaerobic digester bladder, it is to beappreciated that such portable biogas storage containers may be usedwith other anaerobic digester systems.

For a gas distribution set-up, large digesters are required withmultiple biogas storage containers for transfer to customers. The one ormore external biogas storage containers 24 and 240 may be stored in asupport platform 84 as shown in FIG. 7 which rests on a concrete flooror the like in order to support the containers. The support platform 84may have flexible supports 86 for substantially maintaining the biogasstorage container(s) 24 in an upright position. For the exemplary biogasstorage containers shown in FIG. 6, the openings may be circular tocorrespond to the substantially cylindrical shape of the biogas storagecontainers. It is to be appreciated that when other shapes are used forthe external biogas storage containers, the shape and size of theopenings in the support platform may be changed to correspond to thoseshapes and sizes. For example, openings in the support platform shown inFIG. 7 may support the plurality of exemplary stacked substantiallyrectangular biogas storage containers shown in FIG. 8. The number ofbiogas storage containers is determined on daily gas use needs and gasoutput from the digester.

In an alternative embodiment as shown in FIGS. 9–9A, the biogas may bestored inside the substantially flexible bladder for use onsite. Whenused in this manner, the system further comprises an inlet basin 88 forreceiving and conveying waste into the bladder and an externaldisplacement tank 90. The external displacement tank 90 is designed tohold waste that is displaced by internal bladder pressure. The inletbasin 88 is typically small, about 1/30 the size of the bladder volume.A pipe 92 from the inlet basin feeds waste into the waste inlet 14 inthe bladder. A displacement tank pipe 94 from the displacement tank mayextend downwardly from an opening in the bottom of the displacement tankthrough the digester effluent outlet 16 and into the bladder as shown inFIG. 9A. The length of the pipe 94 extending into the bladder from thedisplacement tank should be long enough to make a gas seal tosubstantially prevent gas from leaking out as is well known in the art.The displacement tank may be substantially shallow. The volume of thedisplacement tank may be about ⅓ to about ½the volume of the bladder. Asbiogas is produced inside the bladder, the waste is pushed out of thedigester effluent outlet into the external displacement tank and whenbiogas is used, the displaced waste flows back into the bladder throughthe digester effluent outlet. The external displacement tank may beconstructed of concrete, bricks, steel, or geo-membrane materialsdepending on local conditions and cost. The inlet basin and displacementtank may be round, square, rectangular, a combination thereof or othershapes. The displacement tank pipe 94 may be flexible and durable, about1 foot to about 4 feet long. A semi-rigid pipe may extend downwardlyfrom the bottom of the inlet basin to the waste inlet in the bladder.The inlet basin bottom may be set at the wall height level of thedisplacement tank where the displacement tank floor may be set at thetop elevation of the bladder. The inlet basin is represented in FIG. 5as the “collection pit.”

In use, the bladder(s) may be factory fabricated and field installed. Asshown in FIG. 5, the animal waste may be conveyed with a water flushfrom a tip bucket or hose or the like to an inlet basin 88 or othercollection pit from where it may be conveyed by gravity, diaphragm pump96 or the like to the bladder (the so-called “flex bag digester”). Theanimal waste enters the bladder 12 and 120 at the waste inlet 14 andflows through the bladder 12 and 120 and undergoes anaerobic bacterialdigestion before exiting as digester effluent through the digestereffluent outlet 16 for further processing or land application. Thebladder 12 and 120, for primary waste treatment, may be complemented byother structures for secondary and tertiary waste treatments (i.e.further processing). By way of example only, and as shown in FIG. 5, thedigester effluent may be conveyed through gravity or other conveyance toa shallow lined separation basin-step dam 98, then to a two cell linedpond 100 prior to effluent disposal. The shallow lined separation basinand two cell lined pond are exemplary structures and are not limitingwithin the confines of the invention. “Effluent disposal” as used hereinmeans further processed or land applied. The solids in the waste may beremoved prior to conveyance into the bladder or may be removed throughthe one or more sludge drawdown tubes. The biogas generated in thebladder may be used by the one or more biogas use devices (not shown)and/or stored within the bladder equipped with a displacement tankand/or stored in one or more external biogas storage containers.

From the foregoing, it is to be appreciated that the novel anaerobicdigester system and method may simply, reliably, and inexpensively treatanimal waste with biogas production and recovery in warm climates whereminimum temperatures are about 65° F. or higher. Such system and methodhelp control air and water pollution from livestock waste, protectpublic health and offer an opportunity for the waste to be used as arenewable energy resource. The sizing method substantially permitsbiological stabilization of ambient temperature anaerobic digesterstreating dilute animal wastes and other dilute high strength wastes inregions that lie between 35° north and south latitudes The digester maybe factory fabricated and field installed making it particularly wellsuited for the small farm having limited resources.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

1. An ambient anaerobic digester comprising: a substantially flexiblebladder constructed of a reinforced geo-membrane material and having oneor more waste inlets, digester effluent outlets, biogas outlets, andsludge access ports; wherein the substantially flexible bladder is sizedto have a total volume based on a selected latitude between 35° northand south to maintain the greater of a maximum daily volatile solids(VS) loading rate per 1,000 ft³ or a minimum hydraulic retention time(HRT) at 65 degrees Fahrenheit or higher.
 2. The ambient anaerobicdigester of claim 1, wherein the reinforced geo-membrane material has aminimum thickness of about 20 mils.
 3. The ambient anaerobic digester ofclaim 1, wherein the one or more sludge access ports provide an openingto corresponding sludge drawdown tubes within the interior of thesubstantially flexible bladder.
 4. The ambient anaerobic digester ofclaim 1, wherein an internal baffle defines an inlet side of thesubstantially flexible bladder and an outlet side of the substantiallyflexible bladder, the waste inlet and one or more sludge access ports onthe inlet side and the biogas and digester effluent outlets on theoutlet side of the substantially flexible bladder.
 5. The ambientanaerobic digester of claim 1, wherein the substantially flexiblebladder comprises about a 3:1 to about a 5:1 length: width ratio.
 6. Theambient anaerobic digester of claim 1, wherein the substantiallyflexible bladder has a top surface and a bottom surface with the wasteinlet, digester effluent outlet, biogas outlet, and the one or moresludge access ports in the top surface thereof.
 7. The ambient anaerobicdigester of claim 6, wherein the waste inlet and the digester effluentoutlet are on opposite ends of the top surface of the substantiallyflexible bladder.
 8. An improved ambient anaerobic digester system,comprising: a substantially flexible bladder of reinforced geo-membranematerial for anaerobic digestion of animal waste, the substantiallyflexible bladder having one or more waste inlets, digester effluentoutlets, biogas outlets, and sludge access ports; and means forrecovering biogas from the anaerobic digestion of the animal waste,wherein the substantially flexible bladder is sized to have a totalvolume based on a selected latitude between 35° north and south tomaintain the greater of a maximum daily volatile solids (VS) loadingrate per 1,000 ft³ or a minimum hydraulic retention time (HRT) at 65degrees Fahrenheit or higher.
 9. The improved anaerobic digester systemof claim 8, wherein the means for recovering biogas comprises at leastone biogas storage container, biogas use device or a combination thereofin fluid communication with the one or more biogas outlets in thesubstantially flexible bladder.
 10. The improved anaerobic digestersystem of claim 8, wherein the means for recovering biogas comprises adisplacement tank for receiving animal waste displaced by biogasproduced within the substantially flexible bladder.
 11. The improvedanaerobic digester system of claim 10, wherein the displaced animalwaste exits and returns to the substantially flexible bladder throughthe one or more digester effluent outlets.
 12. The improved anaerobicdigester system of claim 9, wherein the at least one biogas storagecontainer comprises a bladder.
 13. The improved anaerobic digester ofclaim 8, wherein the one or more sludge access ports provide an openingto corresponding sludge drawdown tubes within the interior of thesubstantially flexible bladder.